The field of the invention is magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to methods and systems for phase contrast MRI.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear”, or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems. These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then performing a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
MR methods have been developed that encode spin motion into the phase of the acquired signal. These form a class of techniques known as phase contrast (“PC”) MRI methods. Currently, most PC techniques acquire two images, with each image having a different sensitivity to the same velocity component. Images may then be produced by forming either the phase difference or complex difference between the pair of velocity-encoded images. This motion encoding method is used to image flowing blood in what is commonly referred to as phase contrast magnetic resonance angiography (“PC-MRA”).
Phase contrast techniques such as that described in U.S. Pat. No. 6,954,067 have also been used to image flow and provide quantitative measurement of blood flow. In flow imaging the motion encoding gradients used during the scan are sensitive to velocity components in two or three orthogonal directions. From the resulting velocity component images, total quantitative flow images can be produced.
Phase contrast MRI often utilizes a plurality of acquired image datasets, from which reconstructed image magnitudes often highly correlate. In conventional PC image reconstruction techniques, individual magnitude and phase images are reconstructed first. Then, the reconstructed images are processed to produce both averaged image magnitudes and quantitative parameters from the phase images.
Phase contrast velocimetry MRI can be utilized to provide both quantitative velocity and anatomical information. Velocity information is of particular interest as it can be processed to yield diagnostic measures not generally available from standard contrast developed from spin relaxation. In vascular imaging, PC velocity data can be utilized to determine abnormal flow rates, determine vessel wall compliance, and observe abnormal flow dynamics. Additionally, velocity data can be processed to determine hemodynamic parameters such as wall shear stress and relative pressure. Accurate quantification of velocity fields and parameters derived from those fields often requires images that simultaneously have high temporal and spatial resolution, which is difficult to achieve due to the scan time increase from multiple measurements required for velocity encoding.
Stronger, faster imaging gradients can be utilized to provide limited acceleration to PC scan sessions; however, the acceleration comes at the cost of increased eddy-current artifacts, reduced signal-to-noise ratio (“SNR”), and reduced data acquisition efficiency. Improved acquisition efficiency at high bandwidth can be gained by employing non-Cartesian sampling patterns, such as spiral and radial acquisition schemed. These acquisitions can be used to maintain a similar readout gradient duration as lower bandwidth Cartesian acquisitions, whose length is ultimately limited by T*2 decay, and simultaneously covering more k-space per unit time. These techniques open up the possibility of clinical applications of PC that would not be generally practical, such as real-time 2D or 3D PC MRI.
However, the utilization of phase contrast in areas with significant fat signal surrounding vessels presents a unique set of challenges. In these vessels, chemical shift artifacts can cause fat signal to interfere with neighboring water signal, thereby reducing the accuracy of velocity measurements within those voxels. This problem is especially true when phase contrast is used in conjunction with non-Cartesian trajectories at high field. Unfortunately, limited options are available for reliable fat suppression with phase contrast. Traditional fat-suppression pulses may increase specific absorption rate (“SAR”), reduce available data acquisition time, and suffer from sensitivity to off-resonance effects caused by heterogeneity of the main magnetic field. Selective water excitation can also be utilized; however, in addition to off-resonance sensitivity, these pulses are sensitive to flow artifacts.
Chemical shift imaging (“CSI”) techniques, which acquire images at multiple echo times and utilize the complex signal to separate water and fat, are not sensitive to off-resonance and can produce images with optimal SNR. These techniques can be utilized in conjunction with almost any imaging method, including phase contrast velocimetry; however, they come at the cost of a three-fold increase in the number of measurements required. For interleaved velocity encoding, this also leads to a three-fold increase in the minimum temporal resolution, which is already hampered by the several fold acquisition overhead of phase contrast imaging. The three-fold scan time increase is forced due to the separation of chemical shift imaging from velocity encoding.
It would therefore be desirable to provide a method for phase contrast magnetic resonance imaging that circumvents the need to perform several image reconstructions in order to produce phase images. Moreover, it would be desirable to provide a method for phase contrast velocimetry that further allows for the mitigation of off-resonance and other imaging artifacts without undue burden to the image reconstruction process.